Background:Increased adiposity may trigger signalling pathways that induce aromatase expression. As aromatase inhibitors exert their effects by blocking the aromatase enzyme, higher body mass index (BMI) can reduce the effect of aromatase inhibitors. Thus, we aimed to investigate retrospectively the effect of BMI on the efficacy of aromatase inhibitors in hormone receptor-positive postmenopausal patients with breast cancer.Methods:Newly diagnosed hormone receptor-positive breast cancer patients who were postmenopausal and non-metastatic were enrolled to the study. Patients with BMI ranging between 18.5 and 24.9 kg m 2 were considered as normal weight patients (Arm A, n102), and patients with a BMI ranging 25 kg m 2 were grouped as overweight and obese patients (Arm B, n399).Results:In both normal weight and overweight patients, the baseline clinico-pathologic properties and the treatment history with radiotherapy and chemotherapy were similar, and with no statistically significant difference. In normal weight patients disease-free survival (DFS) rate was 93.7% and 77.6%, whereas in overweight and obese patients DFS rate was 96.8% and 85.5% in the first and third years, respectively, (P0.08). Three year survival rate in Arm A patients was 98.3%, whereas in Arm B was 98.0% (P0.57). When anastrozole was compared with letrozole in the subgroup analysis no difference with regard to DFS and overall survival was detected.Conclusion:These results, contradictory to the prior results, show that BMI has no worse effect on outcomes of aromatase inhibitors in postmenopausal hormone receptor-positive breast cancer patients. In the subgroup analysis, letrozole and anastrozole had similar survival outcomes. © 2012 Cancer Research UK.